More specifically, the invention relates to a control circuit of the type comprising: a MOSFET transistor connected essentially in series with the motor between two terminals of a first D.C. voltage source and having its source terminal connected to the motor; a transistor driving stage, having an input intended to receive a pulse-width modulation (PWM) control signal and an output connected to the gate terminal of the MOSFET transistor, and a bootstrap capacitor connected between a second D.C. voltage source and the source terminal of the MOSFET transistor.
In control circuits of this type where the MOSFET transistor is in a so-called “high side” arrangement, namely one where the drain terminal is connected to the positive terminal of the first D.C. voltage source and the source terminal is connected to the motor, the use of a bootstrap capacitor ensures the necessary conditions for producing, where required, conduction of the MOSFET transistor, keeping the voltage between the gate and source terminals of this transistor at a value of about 10 volts or so, even when the potential of the source terminal tends to rise.
In the case of an electric fan for motor vehicles, and in particular an electric fan for the so-called “radiator set”, the problem arises that, when its impeller is rotated not by the associated electric motor, but by the action of “wind” striking it during forward movement of the motor vehicle (so-called “wind milling effect”), the electric motor operates as a generator and produces at its terminals a voltage which tends to hinder recharging of the bootstrap capacitor, therefore preventing this capacitor from performing its known function described above.